1,381 research outputs found
Dynamics of sliding drops on superhydrophobic surfaces
We use a free energy lattice Boltzmann approach to investigate numerically
the dynamics of drops moving across superhydrophobic surfaces. The surfaces
comprise a regular array of posts small compared to the drop size. For drops
suspended on the posts the velocity increases as the number of posts decreases.
We show that this is because the velocity is primarily determined by the
contact angle which, in turn, depends on the area covered by posts. Collapsed
drops, which fill the interstices between the posts, behave in a very different
way. The posts now impede the drop behaviour and the velocity falls as their
density increases.Comment: 7 pages, 4 figures, accepted for publication in Europhys. Let
Hydrodynamic friction of fakir-like super-hydrophobic surfaces
A fluid droplet located on a super-hydrophobic surface makes contact with the
surface only at small isolated regions, and is mostly in contact with the
surrounding air. As a result, a fluid in motion near such a surface experiences
very low friction, and super-hydrophobic surfaces display strong drag-reduction
in the laminar regime. Here we consider theoretically a super-hydrophobic
surface composed of circular posts (so called fakir geometry) located on a
planar rectangular lattice. Using a superposition of point forces with suitably
spatially-dependent strength, we derive the effective surface slip length for a
planar shear flow on such a fakir surface as the solution to an infinite series
of linear equations. In the asymptotic limit of small surface coverage by the
posts, the series can be interpreted as Riemann sums, and the slip length can
be obtained analytically. For posts on a square lattice, our analytical results
are in excellent quantitative agreement with previous numerical computations
Gravitational oscillations of a liquid column
We report gravity oscillations of a liquid column partially immersed in a
bath of liquid. We stress in particular some peculiarities of this system,
namely (i) the fact that the mass of this oscillator constantly changes with
time; (ii) the singular character of the beginning of the rise, for which the
mass of the oscillator is zero; (iii) the sources of dissipation in this
system, which is found to be dominated at low viscosity by the entrance (or
exit) effects, leading to a long-range damping of the oscillations. We conclude
with some qualitative description of a second-order phenomenon, namely the
eruption of a jet at the beginning of the rise.Comment: 22 pages, pdf. Submitted to Physics of Fluid
A Calibrated Time Domain Envelope Measurement System for the Behavioral Modeling of Power Amplifiers
This paper presents a set-up which enables the generation and the calibrated time domain measurements of complex envelopes of modulated signals at both ports of non linear microwave power amplifiers. The architecture of the characterization tool is given. Examples of error corrected time domain envelopes at the input / output RF ports of a 36 dBm output power – 30dB power gain L-band SSPA are shown. Futhermore, the use of this characterization tool and a suitable processing of measurement data are applied to a novel measurement based behavioral modeling approach of non linear devices accounting for memory effects
A lichen protected by a super-hydrophobic and breathable structure
A species of lichen, Lecanora conizaeoides, is shown to be super-hydrophobic. It uses a combination of hydrophobic compounds and multi-layered roughness to shed water effectively. This is combined with gas channels to produce a biological analogue of a waterproof, breathable garment. The particular lichen grows mostly during wet seasons and is unusually resistant to acid rain [Hauck, M., 2003. The Bryotogist 106(2), 257-269; Honegger, R., 1998. Lichenologist 30(3),193-212]. The waterproof, breathable surface allows this lichen to photosynthesise when other species are covered with a layer of water. In addition, rainwater runs off the surface of the organism, reducing its intake of water from above and probably contributing to its resistance to acid rain
Reshaping and Capturing Leidenfrost drops with a magnet
Liquid oxygen, which is paramagnetic, also undergoes Leidenfrost effect at
room temperature. In this article, we first study the deformation of oxygen
drops in a magnetic field and show that it can be described via an effective
capillary length, which includes the magnetic force. In a second part, we
describe how these ultra-mobile drops passing above a magnet significantly slow
down and can even be trapped. The critical velocity below which a drop is
captured is determined from the deformation induced by the field.Comment: Published in Physics of Fluids (vol. 25, 032108, 2013)
http://pof.aip.org/resource/1/phfle6/v25/i3/p032108_s1?isAuthorized=n
Fracture of a viscous liquid
When a viscous liquid hits a pool of liquid of same nature, the impact region
is hollowed by the shock. Its bottom becomes extremely sharp if increasing the
impact velocity, and we report that the curvature at that place increases
exponentially with the flow velocity, in agreement with a theory by Jeong and
Moffatt. Such a law defines a characteristic velocity for the collapse of the
tip, which explains both the cusp-like shape of this region, and the
instability of the cusp if increasing (slightly) the impact velocity. Then, a
film of the upper phase is entrained inside the pool. We characterize the
critical velocity of entrainment of this phase and compare our results with
recent predictions by Eggers
Wetting on a spherical wall: influence of liquid-gas interfacial properties
We study the equilibrium of a liquid film on an attractive spherical
substrate for an intermolecular interaction model exhibiting both fluid-fluid
and fluid-wall long-range forces. We first reexamine the wetting properties of
the model in the zero-curvature limit, i.e., for a planar wall, using an
effective interfacial Hamiltonian approach in the framework of the well known
sharp-kink approximation (SKA). We obtain very good agreement with a mean-field
density functional theory (DFT), fully justifying the use of SKA in this limit.
We then turn our attention to substrates of finite curvature and appropriately
modify the so-called soft-interface approximation (SIA) originally formulated
by Napi\'orkowski and Dietrich [Phys. Rev. B 34, 6469 (1986)] for critical
wetting on a planar wall. A detailed asymptotic analysis of SIA confirms the
SKA functional form for the film growth. However, it turns out that the
agreement between SKA and our DFT is only qualitative. We then show that the
quantitative discrepancy between the two is due to the overestimation of the
liquid-gas surface tension within SKA. On the other hand, by relaxing the
assumption of a sharp interface, with, e.g., a simple smoothing of the density
profile there, markedly improves the predictive capability of the theory,
making it quantitative and showing that the liquid-gas surface tension plays a
crucial role when describing wetting on a curved substrate. In addition, we
show that in contrast to SKA, SIA predicts the expected mean-field critical
exponent of the liquid-gas surface tension
Comment on “Transition to the Relativistic Regime in High Order Harmonic Generation”
International audienceIn [Phys. Rev. Lett. 98, 103902 (2007)], Tarasevitch et al. demonstrate the existence of two generation mechanisms for laser high-order harmonicsfrom overdense plasmas. One of these mechanisms leads to harmonics with frequencies up to the maximum plasmafrequency of the target and occurs even at nonrelativistic laser intensities. We show that the mechanism responsiblefor these harmonics is coherent wake emission (CWE), a process that significantly differs from thequalitative model proposed by these authors, and it leads toa different interpretation of several essential features of this emission
Can tooth differentiation help to understand species coexistence? The case of wood mice in China
Five wood mice Apodemus species occur across China, in allopatry but also in sympatry up to cases of syntopy. They all share a similar external appearance, similar habitats of grasslands and forests and a generalist feeding behaviour. This overall similarity raises questions about the mechanisms insuring competition avoidance and allowing the coexistence of the species. In this context, a morphometric analysis of two characters related to feeding (mandible and molar) addressed the following issues: (1) Were the species actually different in size and/or shape of these characters, supporting their role in resource partitioning? (2) Did this pattern of phenotypic divergence match the neutral genetic differentiation, suggesting that differentiation might have occurred in a former phase of allopatry as a result of stochastic processes? (3) Did the species provide evidence of character displacement when occurring in sympatry, supporting an ongoing role of competition in the interspecific divergence? Results evidenced first that different traits, here mandibles and molars, provided discrepant pictures of the evolution of the Apodemus group in China. Mandible shape appeared as prone to vary in response to local conditions, blurring any phylogenetic or ecological pattern, whereas molar shape evolution appeared to be primarily driven by the degree of genetic differentiation. Molar size and shape segregated the different species in the morphospace, suggesting that these features may be involved in a resource partitioning between Apodemus species. The morphological segregation of the species, likely achieved by processes of differentiation in isolation promoted by the complex landscape of China, could contribute to competition avoidance and hence explain why no evidence was found of character displacement. © 2012 Blackwell Verlag GmbH
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